Uniaxial strain has recently emerged as a powerful tuning parameter for the unconventional superconductor Sr₂RuO₄. Compressive strain along the [100] direction drives a Lifshitz transition in the Fermi surface, enhancing T_c from 1.5 K to 3.5 K and subsequently inducing a spin density wave (SDW) order. Although this phase diagram is well-established, the evolution of spin fluctuations—critical for understanding the pairing mechanism—has remained unknown.

Here, I will present our recent investigation of paramagnon excitations in the normal state of Sr₂RuO₄ using Ru L₃-edge resonant inelastic x-ray scattering (RIXS) under in-situ uniaxial strain [1]. Under [100] strain, we discovered a momentum-selective softening and enhancement of spin excitations exclusively near the Fermi surface nesting wavevector q = (0.3, 0.3) r.l.u. This clear mode softening firmly establishes the Fermi surface nesting origin of the SDW order.

References

[1] L. Wang et al., unpublished.